Problem: Factor the following expression: $-4$ $x^2+$ $15$ $x+$ $25$
This expression is in the form ${A}x^2 + {B}x + {C}$ . You can factor it by grouping. First, find two values, $a$ and $b$ , so: $ \begin{eqnarray} {ab} &=& {A}{C} \\ {a} + {b} &=& {B} \end{eqnarray} $ In this case: $ \begin{eqnarray} {ab} &=& {(-4)}{(25)} &=& -100 \\ {a} + {b} &=& & & {15} \end{eqnarray} $ In order to find ${a}$ and ${b}$ , list out the factors of $-100$ and add them together. Remember, since $-100$ is negative, one of the factors must be negative. The factors that add up to ${15}$ will be your ${a}$ and ${b}$ When ${a}$ is ${-5}$ and ${b}$ is ${20}$ $ \begin{eqnarray} {ab} &=& ({-5})({20}) &=& -100 \\ {a} + {b} &=& {-5} + {20} &=& 15 \end{eqnarray} $ Next, rewrite the expression as ${A}x^2 + {a}x + {b}x + {C}$ $ {-4}x^2 {-5}x +{20}x +{25} $ Group the terms so that there is a common factor in each group: $ ({-4}x^2 {-5}x) + ({20}x +{25}) $ Factor out the common factors: $ x(-4x - 5) - 5(-4x - 5) $ Notice how $(-4x - 5)$ has become a common factor. Factor this out to find the answer. $(-4x - 5)(x - 5)$